A laminated body has layers of a primary ceramic phase separated by layers of either metal or a secondary ceramic phase. The primary ceramic phase possesses desired properties such as electrical, optical, magnetic, thermal, and/or mechanical properties. For example, the primary ceramic layers may possess superconducting properties thereby yielding a laminated superconductor. As another example, the primary ceramic phase may possess dielectric properties thereby causing the laminate to operate as a multilayer capacitor.
A "ceramic" is defined herein as a compound in which a metal species has been placed in a higher valence state. A metal oxide is a common example of such ceramics. Oxidation is the process by which metal elements are converted to a higher valence state. While metal oxides can contain oxygen, it should be noted that "oxidation" and "oxidized" metals or zones, as those terms are used herein, do not necessarily include oxygen. Examples of oxidized metals include sulfides, halides, carbides, nitrides, carbonates and metal-oxygen compounds. Metals or alloys can be oxidized by heating them in an oxidizing atmosphere (e.g., in atmospheres containing O.sub.2, S.sub.2, N.sub.2, or CO.sub.2). They can also be oxidized by applying an electric potential.
Composites of ceramic and metal of various morphologies are less susceptible to catastrophic brittle fracture than pure ceramics. Several theories have been proposed to explain the improved mechanical properties of a ceramic and metal composite (e.g., crack bridging, crack deflection, crack blunting, and crack shielding). F. Erdogan, P. F. Joseph, "Toughening of Ceramics through Crack Bridging by Ductile Particles," J. Am. Ceram. Soc., Vol 72, No. 2, pp. 262-70, 1989. A. G. Evans, R. M. McMeeking, "On the Toughening of Ceramics by Strong Reinforcements," Acta Metall., Vol. 34, No. 12, pp. 2435-2442, 1986. L. S. Sigl, P. A. Mataga, B. J. Dalgleish, R. M. McMeeking, and A. G. Evans, "On the Toughness of Brittle Materials Reinforced with a Ductile Phase," Acta Metall., Vol. 36, No. 12, pp. 945-953, 1988. According to these theories, the microstructure or morphology of the ceramic-and-metal composite determine the composite's tendency to crack. For example, toughening by a crack-bridging mechanism is enhanced for a metal/ceramic composite consisting of large, interconnected metal particles uniformly distributed about the ceramic phase.
Composites composed of two different ceramics can also be less susceptib1e to catastrophic brittle fracture than single ceramic bodies. Mechanisms for enhanced fracture toughness of ceramic/ceramic composites include transformation toughening, crack-deflection toughening, microcrack toughening, crack-bridge toughening, crack-branch toughening, and fiber-pullout toughening. R. W. Rice, "Ceramic Matric Composite Toughening Mechanisms An Update," Ceram. Eng. Sci. Proc., Vol. 6, pp. 589-607, 1985. R. W. Rice, J. R. Spann, D. Lewis, W. Coblenz, "The Effect of Ceramic Fiber Coatings on the Room Temperature Mechanical Behavior of Ceramic-Fiber Composites," Ceram. Eng. Sci. Proc., Vol. 5, pp. 614-624, (1984); D. B. Marshall, A. B. Evans, "Failure Mechanisms in Ceramic-Fiber/Ceramic-Matrix Composites," J. Am. Ceram. Soc., Vol. 68, No. 2, pp. 225-231, (1985); P. F. Becher, "Microstructural Design of Toughened Ceramics," J. Am. Ceram. Soc., Vol. 74, No. 2, pp. 255-69, (1991).
The morphologies of the primary ceramic and metal phases (or the primary and secondary ceramic phases) of a composite can also play an important role in determining the electrical, optical, thermal, or magnetic properties, in addition to the mechanical properties, of the composite. For example, in an electrically superconducting oxide/normal metal composite, in which the oxide and metal phases are randomly dispersed, the volume fraction of superconducting oxide grains must be sufficiently high to provide a continuous superconduction path. Otherwise, the composite will not exhibit superconductive properties. On the other hand, the fracture toughness of the superconducting oxide/normal metal composite should improve as the volume fraction of the metal phase is increased (or as the volume fraction of the oxide phase is decreased). Thus, a trade-off between superconducting properties and mechanical properties must be made in optimizing the amounts of normal metal phase and superconducting oxide phase in a composite with a random dispersion of metal and oxide phases. A more desirable geometry for a superconducting oxide/normal metal composite is a microlaminate geometry, which is composed of thin, alternating layers of the superconducting oxide and the normal metal. With this geometry, the volume fraction of superconducting oxide phase required for a continuous path of superconduction is less than for a composite with randomly-dispersed oxide and metal phases. Thus, a microlaminate geometry allows for the use of more normal metal (i.e., higher fracture toughness) for the same degree of connectivity of the superconducting oxide phase.
Thus, a laminated composite geometry, composed of alternating layers of a primary ceramic and metal (or a primary oxide and a secondary ceramic) can be a preferred geometry for obtaining a desired combination of electrical, optical, thermal, magnetic, and/or mechanical properties. For example, a microlaminate body composed of alternating planar layers of a thermally-insulating ceramic phase and a normal metal can be thermally-insulating in a direction perpendicular to the planes, while retaining good fracture toughness due to the metallic layers.
One method of producing a composite of an ceramic and metal is to first form a precursor alloy which includes the metallic elements of the desired ceramic and at least one other metal. The alloy is then oxidized to form a composite of the with the other (non-oxidized) metal(s) of the alloy. (See for example, U.S. Pat. No. 4,826,808 "Preparation of Superconducting Oxides and Oxide Metal Composites," by Yurek, et al, incorporated herein by reference). The alloy is formed with a "noble" metal element where "noble" is defined in the sense that the oxidized noble metal is thermodynamically unstable under the reaction conditions employed to oxidize the other elements of the alloy. For example, a noble metal-bearing alloy can be oxidized under conditions that convert certain metallic elements of the alloy to a superconducting oxide without oxidizing the noble metal. The noble metal is exsolved (e.g., precipitated) from the alloy during oxidation as a finely divided, substantially pure metal phase rather than as a second oxide phase. The noble metal phase is thus intimately mixed with the primary oxide phase.
One object of this invention is to provide a laminated ceramic-ceramic composite. Another object is to provide a laminated metal-ceramic composite, in which the ceramic phase contains multiple elements (herein a "multicomponent ceramic"). Another object is to provide a laminated ceramic body that resists mechanical failure. Another object is to provide a laminated ceramic body, in which at least one ceramic phase possesses desired electrical properties (e.g., electrically superconducting, or electrically semiconducting, or electrically metallic, or electrically insulating, or dielectric, or ionically conducting), or desired optical properties (e.g., optically conducting, or optically absorbing, or optically reflecting), or desired magnetic properties (e.g., ferromagnetic, or ferrimagnetic, or paramagnetic), or desired thermal properties (e.g., thermally conducting, or thermally insulating, or thermal-shock resistant), or desired mechanical properties (e.g., fracture-tough ceramics, or strong ceramics), or a combination of the above (e.g., piezoelectric, or pyroelectric, or electrooptic, or acoustooptic, or magnetooptic, or magnetostrictive). Another object includes providing a method of making such laminates that is reliable and avoids complex handling.